travis-ci: Switch to Ubuntu 14.04 'Trusty Tahr'

In order to make other various improvements, update to the latest
environment travis-ci supports.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>